We will investigate thrombocytopenic patients with acute leukemia who require platelet transfusion therapy in order to study prospectively the immunizing effect of frequent and repeated histocompatible or histoincompatible platelet transfusions. Alloimmunization in these patients will be detected by lymphocytotoxic, platelet indirect immunofluorescent, platelet aggregation, and platelet factor 3 immunoinjury assays. These same assays will be utilized to evaluate conclusively the usefulness of platelet crossmatch tests, designed to predict donor-recipient compatibility. In addition, these tests, as well as platelet survival and function measurements, will be made concurrently to study patient refractoriness to histocompatible, histoincompatible, or irradiated platelet transfusions. The role of circulating immune complexes and platelet-specific glycoproteins, such as Beta-thromboglobulin and platelet factor 4, in the refractoriness of patients to platelet transfusion therapies, increased or decreased transfused platelet activation, and transfused platelet survival wil be studied. These investigations on the transfusion of throbocytopenic, leukemic patients should provide much needed information on the relative importance of HLA specificities, platelet-specific antigens, residual lymphocytes in transfused platelets for possible increased activation and clearance of transfused platelets, and immune complex disease in determining the success of platelet transfusion therapy. Such studies promise to place the transfusion of platelets on a more scientific basis, as has already been achieved for red-cell transfusion.